38 PRACTICAL LESSONS IN SCIENCE. 



Again, if a heavier body, as glass or iron, of known weight in 

 air, be weighed in water, the loss of weight shows the volume of 

 an equal weight of water. If the same body be weighed in alco- 

 hol or milk, the loss of weight shows the volume of an equal 

 weight of the milk or alcohol, which may be compared with that 

 of water and the specific gravity found. It is said that the prin- 

 ciple on which the whole subject of specific gravity is based was 

 accidentally discovered by Archimedes, an eminent philosopher, 

 who lived in the third century B. C., who used it to test the purity 

 of a crown, supposed to have been made of gold. 



Liquids not only press in all directions, but they transmit 

 pressure in all directions. Fill a bottle with water, fit it with a 

 good cork, so that the cork touches the water in the bottle; 

 now pressure on the cork will be distributed to every part of the 

 bottle. Insert a pipe into the bottom of a cask and bend it so 

 that the free end is about as high as the cask, then fill the cask 

 with water, and the water will be found as high in the pipe as 

 in the cask. If you pour water into the tube, the water level 

 rises in the cask. If we fit a piston in the pipe and press down 

 upon the water, that force is transmitted to the whole mass in 

 the cask, as the whole mass rises to a higher level. The law 

 may be stated as follows : Pressure exerted on any given area 

 of a fluid, inclosed in a vessel, is transmitted without loss to 

 every equal area of the interior of that vessel. This principle 

 has been fully established by experiment and is utilized in the 

 construction of the hydraulic press. Presses have been made 

 that are capable of lifting as much as 3,000 tons. 



Gases have weight and elasticity, and they surpass liquids in 

 mobility, but they are distinguished for their high degree of com- 

 pressibility and expansibility. Take a test tube fitted with a 

 cork and a delivery tube of about twelve inches in length. Insert 

 the free end of the delivery tube in water or some colored liquid, 

 and apply the flame of the lamp to the test tube until some of 

 the air is forced out; then remove the heat, keeping the delivery 

 tube under the liquid till about one-fourth of it is filled with 

 the liquid; then we have an air thermometer, with which can 



